Add automated public API backward compatibility checks

[erseco]

Summary

• add Roave Backward Compatibility Check to pull-request CI
• compare the public Composer-autoloaded API from origin/main to the PR HEAD
• pin Roave BC Check to version 8.22.0
• run the tool in a dedicated PHP 8.4 job so it does not change the package's PHP 8.0 runtime requirements
• use GitHub Actions annotation output for incompatible changes
• fetch complete git history so revision comparison is reliable
• document the Semantic Versioning/BC policy and intentional-major-release process
• update contributing guidance

Stack

This PR is stacked on #35 and targets security/release-supply-chain.

Behavior

Pull requests that modify src/**, composer.json or the BC workflow fail when they introduce an unapproved public API break relative to main.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.4%. Comparing base (e3554df) to head (a7d8e5b).

Additional details and impacted files

@@                       Coverage Diff                       @@
##             security/release-supply-chain     #36   +/-   ##
===============================================================
  Coverage                             92.4%   92.4%           
  Complexity                             801     801           
===============================================================
  Files                                   30      30           
  Lines                                 2639    2639           
===============================================================
  Hits                                  2441    2441           
  Misses                                 198     198           

Flag	Coverage Δ
unit	92.4% <ø> (ø)

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